Electrophotographic dry developing systems employed in a copying machine, MFP or other types of electrophotographic apparatuses are categorized into a system using a one-component developer containing only toner and a system using a two-component developer containing toner and carrier. In either of these developing systems, toner charged to a predetermined level is applied to a photoreceptor. An electrostatic latent image formed on the photoreceptor is rendered visual with the toner and is transferred to a sheet of paper. The image visualized by the toner is fixed on the paper to obtain a desired image.
A brief description about development with the two-component developer will be given. A predetermined amount of toner and a predetermined amount of carrier are accommodated in a developing apparatus. The developing apparatus is provided with a rotatable magnet roller with a plurality of south and north poles alternately arranged thereon in the circumferential direction and an agitation roller for agitating and mixing the toner and carrier in the developing apparatus. The carrier made of a magnetic powder is carried by the magnet roller. The magnetic force of the magnet roller forms a straight-chain like magnetic brush of carrier particles. Agitation produces triboelectric charges that bond a plurality of toner particles to the carrier particles. The magnetic brush abuts against the photoreceptor with rotation of the magnet roller to supply the toner to the surface of the photoreceptor. Development with the two-component developer is carried out as described above.
Fixation of the toner on a sheet of paper results in successive consumption of toner in the developing apparatus, and new toner in the same amount as that of the consumed toner is supplied, whenever needed, from a toner hopper in the developing apparatus. On the other hand, the carrier is not consumed for development and is used as it is until the carrier comes to the end of its life. The carrier, which is a component of the two-component developer, is required to have various functions including: a function of triboelectrically charging the toner by agitation in an effective manner; insulation properties; and a toner transferring ability to appropriately transfer the toner to the photoreceptor. To improve the toner charging performance, the recent carrier is especially required to have appropriate electric resistance (hereinafter, sometimes simply referred to as “resistance”) and appropriate insulation properties.
From the viewpoint of environmental-friendly copy machine components and the safety issues, some recently devised carriers do not contain heavy metals, such as copper (Cu), zinc (Zn), manganese (Mn), which used to be contained in conventional non-coated carrier. These carriers are composed of, for example, carrier core particles mainly made of soft ferrite and coating resin covering the surface of the carrier core particles. Both the carrier core particles and resin offer toner charging performance and proper insulation properties to the resin-coated carrier. Techniques of manufacturing the carrier core particles of the resin-coated carrier are disclosed in Japanese Unexamined Patent Application Publication Nos. 2007-271663 (PTL 1) and 2007-273505 (PTL 2). The method for manufacturing the carrier core particles shown in PTL 1 and PTL 2 will be briefly described. First, slurry is prepared from raw materials, such as magnetite (Fe3O4), and then the slurry is granulated into carrier core particles by a spray dry method or other methods. Second, the granulated material is fired to urge reduction reaction thereof to form a magnetic phase. Third, the granulated material is disintegrated and classified to obtain granules of a desired particle size. At last, the granules are oxidized.